A rechargeable battery is normally used to supply power in mobile radios. Such rechargeable batteries have an operating voltage of, for example, 3.3 volts to 4.2 volts, depending on their state of charge. In addition, rechargeable batteries such as these, such as lithium-ion rechargeable batteries, metal-hydride rechargeable batteries or nickel-cadmium rechargeable batteries, have a relatively high internal resistance, typically of around 200 milliohms.
The trend for integration of digital camera functionality in mobile telephones is leading to the requirement to also integrate high-power flash appliances in mobile radios.
By way of example, it would be possible to fit so-called xenon flash lamps in order to manage with the given, maximum current driver capability of the rechargeable battery. However, these have relatively high operating voltages of, for example 300 volts, which require additional protective measures against live parts being touched, because these voltages considerably exceed the low-voltage protection limits as specified in many countries. This would be associated with a relatively high degree of complexity.
Light-emitting diodes may likewise have a flash function without having to exceed the low voltage protection limit of 60 volts to do so. However, one problem in this case is that light-emitting diodes such as these require relatively high currents of up to 5 amperes as a peak load with a forward voltage of 5 volts in order to achieve adequate flash brightness. However, the types of rechargeable battery mentioned above cannot provide such high currents because of their relatively high internal resistance, even just for a short time period, since the collapse of the voltage would cause the mobile telephone to reset itself.
Because of the limited energy content of rechargeable batteries of a predetermined weight and volume, it is desirable to achieve high load efficiencies in mobile appliances.